function check_form(form) {
   	if (form.password.value == "" && form.username.value == "") {
   		alert("账号密码为空");
   		return false;
   	}
   	if (form.username.value == "") {
   		alert("账号为空");
   		form.username.focus();
   		return false;
   	}
   	if (form.password.value == "") {
   		alert("密码为空");
   		form.password.focus();
   		return false;
   	}
   	if (form.password.value.length < '8') {
   		alert("密码小于8位");
   		form.password.focus();
   		return false;
   	}
   	return true;
   }
